How Sleeve-valve Engines Work

October 1945: An obsolete Junkers JU 88 transport plane with a Focke-Wulf FW 190 fighter on top, at a display of British and German aircraft at the Royal Aircraft Establishment in Farnborough, England. Take a look at our animation of how the sleeve valve engine works.

During World War II, engineers within the Nazi regime devised some of the best and most-advanced aerial weaponry of the era. One German fighter plane, the Focke-Wulf Fw 190, for a time outperformed anything the Allies could put in the air.

Fortunately for the Allies, engineering on their side eventually swung the air superiority pendulum to their advantage. A rugged, unconventional engine that many people today have probably never even heard of helped to neutralize the Fw 190 and the rest of the Luftwaffe. In its own way, an engine helped propel the Allies to victory [source: Rickard].

The sleeve-valve engine, which has been used on both automobiles and airplanes, powered speedy British fighters such as the Hawker Typhoon and Hawker Tempest. With their brute horsepower, they helped the Allies control the skies, provide air support for ground forces and eventually win the war.

But what exactly is a sleeve-valve engine, and what's with the funny name? And why don't we see or hear much about them today?

The engine gets its name from the thin-walled, metal sleeve that slides up and down within each cylinder during the combustion process. Typically, holes in the sleeve and in the cylinder containing it line up at predictable intervals to expel exhaust gases and suck in fresh air.

Despite its honorable armed services record, the complex sleeve valve setup lost out to what we use in internal combustion engines today, tappet valves. In airplanes, of course, piston-driven powerplants of all types largely gave way to jet engines.

But hold on -- don't dismiss the sleeve valve as a useless historical relic just yet.

At least one company is seeking to bring the venerable sleeve valve engine back into action, but with a few modern twists.

In the next few pages, we'll take a look at just what makes the sleeve-valve engine turn. We'll also examine why it fell out of favor, along with the reasons it's being called up now, more than a century after its invention, to serve in a different kind of "fight."

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Sleeve-valve Engine Technology

Arriving as it did during the height of the Industrial Age, the sleeve-valve engine looks like a contraption that would be right at home in a steampunk novel. Modern-day engineers marvel at its cleverness. And cluck-cluck at its high complexity.

So there, you've been warned. Actually, it's a pretty beautiful thing once you understand how all those pieces work together. Now roll up your sleeves, because we're about to get down and dirty with the inner workings of a sleeve-valve engine.

This engine has so much going on that it almost defies description. But we'll try. Sleeve-valve engines, like their tappet valve counterparts, can come in many different configurations. One such arrangement, the radial sleeve-valve engines used on airplanes, look a bit like what you might get if a Rock 'Em Sock 'Em Robot had a baby with a "squiddie" sentinel from "The Matrix."

To understand what a sleeve-valve engine is and does, it might help to first understand what it is not. It is not, first and foremost, the popular system with which most of us are familiar, a poppet valve engine. Poppet valves are the de facto standard on today's internal combustion engines. With them, mushroom-shaped valves under the tension of springs open and close rhythmically to control the entry and exit of fuel, air and waste exhaust gases in the cylinder.

A sleeve valve, on the other hand, uses a sliding, sometimes rotating sleeve to control how much air and fuel get detonated with each compression stroke. The basic premise of igniting fuel and air to drive a set of pistons and turn a crankshaft is the same as it is with other internal-combustion engines.

Here's another distinct feature of sleeve valves. On designs where the sleeve rotates, ports that are cut into it align with either intake ports or exhaust ports in the cylinder, depending on what part of the stroke is taking place. A piston moves up and down within each sleeve, even as the sleeve is sliding back and forth. The sleeve motion is driven by gears connected to the crankshaft.

Scratching your head still on what, exactly, takes place? Here are the steps:

Compression stroke: the piston approaches top-dead-center, all of the cylinder's ports are closed, and the spark plug fires and ignites the fuel/air mix

Combustion stroke: ignition forces the piston back down into the cylinder; as the piston goes to bottom-dead-center, the liner (or sleeve) shifts to align its cutout openings with the cylinder's exhaust ports

Exhaust stroke: exhaust gas is expelled as the piston comes back up; the exhaust ports close

Intake stroke: the sleeve rotates the other way, exposing the air intake ports; the piston descends, drawing in fresh air; the sleeve shifts to close off the intake port for the next firing stroke and then the entire process repeats

Now multiply that by several cylinders and toss in a crankshaft for them to rotate, and you've got yourself a sleeve-valve engine!

If it sounds complicated, well, that's because it is. One of the main knocks against these engines was that they were so complex. It makes a bit more sense, though, when you see the entire process in action. Check out the video on this page to better visualize it.

Get Your Swirl On: Sleeve Valves and Volumetric Efficiency

So why would anyone want to monkey around with an engine this complicated? After all, they were notoriously thirsty for lubricating oil; and they didn't take kindly to impurities such as grit. The answer is that they offer the advantage of volumetric efficiency. In other words, they're much better than regular engines at getting air into and out of the combustion chamber. Also, the arrangement of the ports provide better swirl characteristics. That's engineer-ese for, they create turbulent air, causing the air and fuel mix to burn more efficiently [source: Raymond].

Sleeve Valves by Land -- Use in Automobile Engines

Indiana-born Charles Yale Knight purchased a three-wheeled Knox automobile around 1901 so that he could report and publish his farm journal in the U.S. Midwest. But he found the clatter created by the car's valves to be a serious pain in the ears. So he did what any self-respecting entrepreneur with a background in industrial machinery would do: He set out to build a better engine himself.

With a wealthy backer's support, he developed and extensively tested prototypes. By 1906, he had made enough progress to reveal his 4-cylinder, 40-horsepower "Silent Knight" car at the Chicago Auto Show.

The Knight engine featured not one, but two sleeves per cylinder, with the inner sleeve sliding within the outer. The piston, in turn, slid inside the inner sleeve. The Knight, true to its moniker, was impressively quiet. Even though the Knight engine proved superior to the loud and fragile poppet valves of its day, U.S. automakers gave it the cold shoulder, initially.

Knight and his financial benefactor L.B. Kilbourne fared considerably better overseas. After some refinements to the design, the Knight engine found its way onto Daimler cars in England (not to be confused with Daimler-Benz).

The Silent Knight was a hit, and soon other manufacturers wanted in on the sleeve valve action -- including automakers in the United States. Willys cars and light trucks, Daimler, and Mercedes-Benz, among others, employed the Knight sleeve-valve engine [source: Wells].

However, by the 1920s, sleeve valve design had advanced beyond Knight's sleeve-within-a-sleeve configuration. Single-sleeve designs, including the Burt-McCollum, were lighter, less complex and less costly to build, and therefore preferable to manufacturers. With further modification from engine manufacturers such as Bristol and Rolls-Royce, they would even take to the sky.

Sleeve Valves by Air -- Use in Airplane Engines

Harry R. Ricardo (later "Sir" Harry Ricardo), born in London in 1885, didn't wait until college to begin his engineering studies. He observed and absorbed at the knee of a local machinist as a young boy, and would go home from the machinist's shop to apply his new knowledge in building engines. He would later say:

"As a child, I was always fascinated by engines and mechanical motions generally, and above all, by the great mystery as to how such things were actually made...looking back, I think I learnt more of actual value from these early and very crude attempts at design and manufacture than from anything else" [source: University of Cambridge].

Ricardo, in his working engineer adulthood, was an incurable overachiever. In addition to tweaking the engines on tanks that helped break the stalemate of World War I, he led ground-breaking research into assigning octane ratings to different grades of fuel.

Perhaps his most notable contribution in the World War II years was his work on making the sleeve-valve engine better.

Ricardo theorized in the 1920s that a sleeve-valve airplane engine could generate greater horsepower than a comparable tappet-valved engine because it could generate a higher compression ratio.

It so turned out that by 1941, British aircraft including the mainstay Supermarine Spitfire fighter plane, were taking a pounding from Germany's superior Focke-Wulf Fw 190. The Fw 190s also launched ground attack raids on Allied installations with near-impunity, since nothing could catch them at low altitude after they dropped their bombs.

The sleeve valve-engined Hawker Typhoon, entering service in 1942, changed that. Propelled by a 2,180-horsepower Napier Sabre engine, the "Tiffy's" extra get-up-and-go meant it could not only shoot down quick Luftwaffe interlopers, but it could carry bombs as well. Later in the war, bomb- and rocket-equipped Typhoons would prove pivotal in supporting Allied ground forces as they tightened the noose on the Nazis and ended the war in Europe [source: Rickard].

Despite the sleeve-valve engine's exemplary military record, the writing was on the wall: jet engines would dominate commercial and military aviation from the postwar years forward.

The legacy of Knight, Ricardo and others would not completely go away -- engine enthusiasts would memorialize the sleeve-valve engine with home-built models and on Web sites in the decades to follow. Some flying model planes use miniature sleeve-valve engines. And it's conceivable the technology could experience a resurgence in some of the world's largest and fastest-growing automotive markets.

What's next?

So, was the sleeve-valve engine an evolutionary dead end, as far as the advance of internal combustion?

Let's put it this way. Just like Hollywood likes to recycle old concepts and put a fresh spin on them when it's running low on new ideas, so does the auto industry. Electric cars, you may recall, were a big deal before (ironically) the electric starter made internal combustion cars highly practical. Electrics pretty much vanished from mainstream motoring until environmental concerns brought them back from the grave near the turn of the century.

And so, similarly, could the case be unfolding with the slumbering sleeve-valve engine. As the saying goes, "what's old is new again."

San Carlos, Calif.-based Pinnacle Technologies is counting on pent-up demand for clean, cheap transportation in Asia to snap up its modern interpretation of the sleeve valve. A new engine is based on what the company describes as a four-stroke, spark-ignited (SI), opposed-piston, sleeve-valve architecture.

"This engine technology provides the fuel economy and CO2 emissions of a hybrid at a price that the whole world can afford," Cleeves said in a company-issued statement

Pinnacle says it isn't worried about electric vehicles making its technology obsolete any time soon. Instead, it believes there's a big opportunity to serve rapidly growing markets such as India and China. They and other developing countries want to curb greenhouse gas emissions while improving their citizens' standard of living, through motor vehicle ownership. Since electric vehicles and hybrids still carry a significant price premium, Pinnacle says its re-envisioned sleeve-valve is a good "bridge technology" until electrics become more affordable for everyone.

Pinnacle, which has received several million dollars in venture capital, said it was pursuing a licensing agreement with an Asian auto manufacturer and it expected production to begin in 2013.

Author's Note: How Sleeve-valve Engines Work

As a big military aircraft geek, I had heard of sleeve-valve engines prior to this assignment. But that was about the extent of it. Given their footnote-in-history status, I had always thought of them merely in the abstract. Unlike a poppet valve engine that you can study in your own driveway, these "sleeve-valve things" were to me just a forgotten, if quaint, technology, like steam locomotives. So when I tapped the power of the Interwebs to see them in action, I was instantly struck with both awe and admiration. How did folks 100 years ago figure out all the necessary angles, tolerances, weight balances and more to bring these incredibly complex machines to life? The fact that entrepreneurs today are looking to breathe new life into the concept speaks volumes about those original pioneers' genius and vision. One could argue that the original, twentieth-century sleeve-valve engines were "over-engineered" -- that is, they were too complicated for their own good. Or it could simply be that, lacking the advances in materials science and the precision of computer-aided design that we enjoy today, they were merely ahead of their time.